A soft handover refers to a situation where a terminal equipment simultaneously utilizes radio resources of two or more base stations for transfer of information. These base stations are referred to as an active set of base stations. In addition to active base stations, the coverage area of a terminal equipment can also comprise passive base stations, which are not used for information transfer due to the lower field strength they provide. SSDT (Site Switching Diversity power control) is an embodiment of soft handover, where active base stations, with which a terminal equipment communicates, are divided into primary and secondary base stations. Transfer of information between the radio network, the base stations and the terminal equipment takes place via the primary base stations, whereas information transfer to the secondary base stations is blocked over the radio connection, excluding control channels. In an ESSDT (Enhanced SSDT) method, transfer of information to the secondary base stations is also blocked on the control channels.
Base stations are divided into primary and secondary base stations such that the terminal equipment measures quality characteristics of pilot signals transmitted by all the base stations of the active base station set with which it has set up a connection. The measurement result reported by the terminal equipment to the base stations consists for example of the identity of the base station providing the best signal quality. The base station compares the identity received from the terminal equipment to its own identity, and if the identities are identical, the base station allows information transfer over the radio link. When the identities do not match, the base station blocks the transmission of information. The base station also compares the quality of the received signal to a preset quality threshold. If the quality of the received signal does not exceed the quality threshold, transmission of information between the base station and the terminal equipment is allowed even though the received identity did not match the identity of the base station. Information transmitted over different physical channels and via different cells is forwarded to L2 protocol functions via a combining-splitting point of the radio network, which implements C/S (Combining/Splitting) functionality.
As is well known, the status of the radio link, i.e. either the primary or secondary status of the base station, does not affect information transfer between the base stations and the combining-splitting point, but information transfer is activated regardless of the base station status. A maximum time determined for a change of base stations for example in WCDMA (Wideband Code Division Multiple Access) standards is 10 ms, in which time a base station should be able to either activate or deactivate transfer of information over a downlink radio link on the basis of the measurement result.
A drawback of the prior art arrangements is that information transmission capacity is wasted between the combining-splitting point and the base stations if the primary/secondary status of the base stations is not taken into account.